The present invention relates to a heater having a positive temperature coefficient resistor as a heating element, particularly relates to a heater for an air-fuel mixture for improving starting and operating performances of engines by heating an air-fuel mixture susceptible to internal combustion engine and promoting evaporation of fuel.
Hitherto, it has been proposed to improve an ignition and combustion of an air-fuel mixture in the internal combustion engine so as to stabilize the operation thereof by heating an air-fuel mixture supplied to the internal combustion engine and promoting evaporation of fuel. As heaters, therefore, there has been known such one that is made by arranging a honeycomb-like positive temperature coefficient resistor between a carburetter and an intake manifold as known in U.S. Pat. No. 4,108,125, or such one that is made by arranging a heater fitted a cylindrical positive temperature coefficient resistor coated with electrode layers on inner and outer surfaces thereof within a ceramic cylinder in an intake manifold as known in Japanese Patent Laid-open No. 75,637/79, or such one that is made by arranging a heater provided with a plate-like positive temperature coefficient resistor housed in the container and secured to a heat dissipating cover on the wall of an intake manifold just under a carburetor, as known in U.S. Pat. No. 3,987,772.
The heater composed of a honeycomb-like positive temperature coefficient resistor, however, is susceptible to mechanical shocks and thermal shocks, since their positive temperature coefficient resistor is directly contacted with the air-fuel mixture, so that the electrode of said positive temperature coefficient resistor becomes to be corroded by fuel ingredient or moisture contained in the air-fuel mixture, and the positive temperature coefficient resistor is reduced. As a result, even if the electric insulating coating is applied on the surface of said positive temperature coefficient resistor, the heat transmission property from said positive temperature coefficient resistor to the air-fuel mixture is not only deteriorated, but also the swelling or peeling of coating material is caused, and the heater cannot withstand the use for a long period.
Moreover, in the heater arranged in the cylindrical positive temperature coefficient resistor and the ceramic cylindrical body, the breakdown is caused by thermal stress generated by a thermal expansion difference between both the bodies, the dimensional precision is required in fitting, and the wall surface in the intake manifold should mechanically be worked because of arrangement in the intake manifold. Thus, this heater is economically disadvantageous. In addition, the heater made by housing the positive temperature coefficient resistor secured with the heat dissipating cover in the container is arranged in the wall of the intake manifold just under the carburetor. However, said intake manifold is provided with a fluid passage for heating the air-fuel mixture by the exhaust gas or for cooling the internal combustion engine by cooling water, so that the position for securing the heater is limited and the intake manifold becomes complicated in design, and as a result, this heater becomes expensive in construction and poor in practice.
This invention is made for eliminating the above drawbacks of a heater for heating an air-fuel mixture with the use of these conventional positive temperature coefficient resistors as a heating element.